In recent years, with the development of a material having corrosion resistance under high temperature and high pressure as a backdrop, high pressure acid leaching with sulfuric acid has been attracting attention as hydrometallurgy of nickel oxide ores (for example, refer to Patent Literature 1). Unlike pyrometallurgy, which is a conventional common method for smelting nickel oxide ores, the high pressure acid leaching does not comprise pyrometallurgical processes, such as reduction and drying, but comprises an integrated hydrometallurgical process, and thus is advantageous in terms of energy and cost. In other words, the above-mentioned high-pressure acid leaching has a great advantage because, in a leaching process, the oxidation reduction potential and temperature of a leachate in a pressure leaching reactor are controlled, thereby fixing iron, a main impurity, to a leaching residue in the form of hematite (Fe2O3), whereby nickel and cobalt can be selectively leached over iron.
For example, as hydrometallurgy of nickel oxide ores, high-pressure acid leaching by using an autoclave has been adopted, wherein, in a high-pressure acid leaching process in which that a raw material slurry is leached by using an autoclave under high temperature and high pressure and then the temperature and pressure of the leached slurry is reduced to normal temperature and pressure by using the flash vessel, a liquid level in the flash vessel is usually measured by a sensor directly attached to the flash vessel.
Here, as shown in FIG. 5 illustrating a schematic structure of a common flash vessel 100, the flash vessel 100 comprises a body portion 101 in the form of a closed-bottom cylinder; a slurry inlet 103 and a steam outlet 105 are provided at a ceiling portion 102 which closes the open upper part of the body portion 101; and a slurry outlet 104 is provided at the body portion 101. A slurry feeding pipe 113 configured to feed a leached slurry of which the temperature and pressure is reduced to a predetermined temperature and a predetermined pressure (hereinafter, sometimes simply referred to as a slurry) into the flash vessel 100 is coupled to the above-mentioned slurry inlet 103. A slurry discharge pipe 114 configured to discharge the slurry fed into the flash vessel 100 is coupled to the above-mentioned slurry outlet 104. A steam discharge pipe 115 configured to recover steam which is generated inside the flash vessel 100 with feeding of the slurry is coupled to the above-mentioned steam outlet 105. A slurry discharge valve 116 is attached to the slurry discharge pipe 114 coupled to the slurry outlet 104.
Furthermore, a leached slurry of which the temperature and pressure is reduced to a predetermined temperature and a predetermined pressure (hereinafter, sometimes simply referred to as a slurry) is fed into this flash vessel 100 via the slurry inlet 103, and the slurry fed into the flash vessel 100 is discharged from the slurry outlet 104, meanwhile steam which is generated with the feeding of the slurry is discharged from the steam outlet 105.
At this time, measurement results of a liquid level in the flash vessel measured by liquid level sensors 120A and 120B are used to keep a liquid level in the flash vessel 100 at a proper liquid level.
For example, in the case where a liquid level is measured with the liquid level sensors 120A and 120B which are installed at upper and lower limits of the liquid level, respectively, when a liquid level has risen and the liquid level sensor 120A installed at the upper liquid level limit detects the liquid level accordingly, the above-mentioned slurry discharge valve 116 is opened to discharge a slurry remaining in the flash vessel 100, whereas when a liquid level had dropped to the extent that the liquid level sensor 120B installed at the lower liquid level limit cannot detect the liquid level, the above-mentioned slurry discharge valve 116 is closed to stop discharging of a slurry from the flash vessel 100. Consequently, a slurry liquid level in the flash vessel 100 is controlled to be between the above-mentioned upper and lower limits. Furthermore, in the case where a liquid level is continuously measured, when a liquid level rises to be higher than a control liquid level, the slurry discharge valve 116 is opened wider, whereby the discharge amount of a slurry remaining in the flash vessel 100 is increased, whereas, when a liquid level falls to be lower than the control liquid level, the slurry discharge valve 116 is opened smaller, whereby discharging of a slurry from the flash vessel 100 is controlled.
Generally, a leaching reaction in the above-mentioned high-pressure acid leaching process is controlled by, other than temperature, a control factor (pH, oxidation-reduction potential) of the leaching reaction by using a leaching agent. For example, a leaching method using chlorine gas as a leaching agent is performed by using an oxidation-reduction potential in a leachate, and therefore a pressure in an autoclave is not directly controlled, and not stable or fixed during leaching operation, and it varies depending on injection amount of chlorine gas which is controlled by the oxidation-reduction potential.
In the case where a leaching agent is a liquid and no gas is generated by a reaction, generally, a pressure in an autoclave is produced by saturated vapor pressure depending on temperature. For example, in recent years, high-pressure acid leaching by using an autoclave has been adopted as hydrometallurgy of nickel oxide ores in order to recover valuable metals, such as nickel and cobalt.
According to the above-mentioned high-pressure acid leaching, for example, in an ore treatment process, an ore slurry having a predetermined slurry concentration and containing ores having a size of 2 mm or less is first prepared by using an pulverizing apparatus and a sieving apparatus. Then, the above-mentioned ore slurry is fed into a high-pressure acid leaching process. Here, the above-mentioned ore slurry is gradually heated and pressurized with a preheater (an apparatus to increase temperature and pressure), and then fed into an autoclave. In the autoclave, impurity elements, such as iron, aluminum, and zinc, as well as nickel and cobalt which are contained in the ores are partly leached with sulfuric acid, whereby a leached slurry containing these is obtained. Subsequently, the above-mentioned leached slurry is transferred from the autoclave to a flash vessel configured to reduce the temperature and pressure of the leached slurry to normal temperature and pressure, and the temperature and pressure of the leached shiny is gradually reduced. After that, the leached slurry goes through a preneutralization process for neutralizing free sulfuric acid in a leachate, a solid-liquid separation process by using thickeners with a multi-stage type, and the like, thereby being separated into a leaching residue and a leachate.
Here, the adoption of the flash vessel in the above-mentioned high-pressure acid leaching process fills a gap between an operating condition of an autoclave in the high-pressure acid leaching process and an operating condition of the subsequent process. In other words, as a leaching condition of the autoclave, a temperature of approximately 200 to 300 degrees C. is usually selected in order to achieve a high leaching rate of nickel and cobalt. On the other hand, from a safety and economic standpoint, the subsequent preneutralization process or solid-liquid separation process is usually operated under atmospheric pressures. Therefore, while gradually recovering pressurized steam from a leached slurry having a high temperature and a high pressure, a flash vessel reduces the temperature and the pressure of the leached slurry.
Here in the high-pressure acid leaching process, a very expensive pipe which is made of a material and has a structure to be resistant to high temperature and pressure is used as a pipe to transfer the leached slurry from the autoclave to the flash vessel, a pipe to supply the above-mentioned recovered steam to the preheater for the ore slurry, a pipe to gradually increase the temperature and pressure of the ore slurry, and the like; and, based on requests concerning overall costs including materials costs, piping is made short as much as possible and each equipment is appropriately arranged. Thus, the leached slurry is transferred from the autoclave to a first-stage flash vessel, and further transferred to another subsequent-stage flash vessel one by one. Here, as a method for transferring the leached slurry between the flash vessels, not a mechanical transfer method like a pump, but a method of transferring the slurry by making use of a height difference between positions of installing flash vessels and a pressure difference between each stage is usually adopted. This is because sulfuric acid is contained in the leached slurry and thus durability and costs of transfer equipment have been taken into consideration. For example, in a practical plant, in the case where an autoclave having a size of approximately 4 to 6 m in diameter and approximately 25 to 30 m in length and having a cylindrical shape is horizontally installed, the first-stage flash vessel is installed at a position located at approximately 25 to 35 m height above the autoclave.
The pressurized steam gradually recovered from the leached slurry having a high temperature and a high pressure is fed into a preheater having the same level of a temperature and pressure as those of the slurry from each stage of the flash vessels, and, as mentioned above, also for this piping here, a very expensive pipe which is made of a material and has a structure to be resistant to high temperature and high pressure is provided.
However, problems about the occurrences of damage and the like to the steam discharge pipe, the slurry discharge pipe and the slurry discharge valve have not been completely solved out, and approximately ten occurrences of damage to the steam discharge pipe and the slurry discharge valve have taken place per year operation, and therefore a practical technique to further reduce the problems about the occurrence of damage has been required.
A cause about the above mentioned damages is presumed to lie in insufficient control of liquid level. In other words, at the time when a leached slurry having a high temperature and a high pressure is fed into a flash vessel and steam is generated accordingly, a liquid surface of the slurry is presumed not to be flat, or rather, it is presumed that steam generated from a deep part of the slurry causes a liquid level thereof to violently vary, whereby the control of the liquid level is insufficient.
In other words, there is only the above-mentioned presumption because, in the high-pressure acid leaching process adopted as hydrometallurgy of nickel oxide ores and using an autoclave, a flash vessel configured to reduce the temperature and pressure of a slurry obtained after leaching a raw material slurry under high temperature and high pressure by using an autoclave has a large size and furthermore is to be applied to a strongly acidic slurry, and therefore it is technically hard to provide an observation window to the flash vessel, and therefore it is substantially impossible to visually observe the inside of the flash vessel.
The conventional flash vessel 100 may have a problem that, for example, even if a liquid level is actually high, the liquid level sensor 120A installed at the upper limit liquid level cannot detect the liquid level due to violent fluctuation of a liquid surface, and the slurry discharge valve 116 does not control the liquid level accordingly, whereby operation continues to be performed in such a state that the liquid level in the flash vessel 100 is high, and accordingly an acidic slurry is transferred together with recovered steam to a preheater, and due to the acidic slurry corrosion of the recovered steam discharge pipe 115 is advanced. On the other hand, the prior flash vessel 100 may have another problem that, even if a liquid level is actually low, similarly the liquid level sensor 120B installed at the lower limit liquid level cannot detect the liquid level accordingly, and the shiny discharge valve 116 does not control the liquid level, whereby the liquid level actually becomes lower than the position of the slurry discharge pipe 114, and steam in the flash vessel 100 is discharged together with a discharged slurry from the slurry discharge pipe 114 to the subsequent-stage flash vessel, whereby a flow rate of the slurry in the discharge pipe is made temporarily higher, thereby causing damages of the slurry discharge pipe 114 and the valve, and the amount of steam flowing from the subsequent-stage flash vessel to the recovered steam pipe is temporarily increased, thereby causing a more amount of the acidic slurry to be transferred, or the increase in the flow rate causes corrosion and wear of the recovered steam pipe to advance.
For example, Patent Literature 2 discloses a method for condensing a slurry of organic sludge, wherein a liquid surface in a flash vessel is detected, whereby the liquid level of a condensed liquid is always kept higher than the position of an outlet. However, the method is hard to directly apply to solve the above-mentioned problems because conditions are too different, that is, a target to be treated is a slurry of organic sludge, the steam pressure is 2.5 atmospheres at most, and the like.
Furthermore, for example, Patent Literature 3 discloses a technique for controlling the refrigerant charge into a refrigerant vapor compression system by use of at least one sensor which detects the level of a liquid refrigerant in a flash vessel used for the system, but the technique is used for a float type sensor, a supersonic sensor, or the like as a sensor, and accordingly is applicable only when a liquid surface is flat, and hence the technique is hard to apply to solve the above-mentioned problems.